Vibrational dephasing of CH stretching vibrations in compounds containing the CH 3 or the CCH group

Abstract

Calculations are presented for the dephasing of the symmetric methyl stretching vibration taking account of the internal rotation of the methyl group in a rigid molecular environment of carbontetrachloride molecules, using two different liquid state models. The narrowing effect of the fast internal rotation on the bandshape is demonstrated. The Raman isotropic spectrum of the methyl stretching vibration in hexadiyne-2,4 is presented, and its second moment and bandwidth are shown to be represented fairly well by the calculations. The acetylenic CH stretch of 3,3-dimethylheptyne-1 has also been investigated. After correction for a hot band on the low frequency side its second moment and bandshape may be reasonably well well reproduced by calculations in the slow modulation limit. The implications for the modulation mechanism of repulsive and attractive forces in liquids are discussed.